This application claims the benefit of a Japanese Patent Application No.2002-060942 filed Mar. 6, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium and a magnetic storage apparatus which are suited for high-density recording.
2. Description of the Related Art
Recording densities of longitudinal magnetic recording media used in magnetic storage apparatuses such as magnetic disk units have greatly increased, due to reduction of media noise and development of magneto-resistive heads and spin-valve heads. In order to reduce the media noise of the magnetic recording medium, it is essential to further reduce the grain size of a magnetic layer which is formed by a collection of fine magnetic particles, and to reduce the magnetic coupling strength between the grains of the magnetic layer.
As the grains of the magnetic layer become magnetically isolated, the recording state is disturbed with lapse of time, thereby causing thermal instability. The thermal instability occurs when the magnetic orientation of some of the grains which should originally be oriented in the axis of easy magnetization is thermally disturbed and deviates from the axis of easy magnetization.
In order to reduce the thermal instability, it is necessary to increase the magnetization energy (anisotropic energy) along the axis of easy magnetization or, to increase the grain size so as to increase the volume energy. However, magnetic inversion becomes difficult when the anisotropic energy is increased, and an upper limit of the anisotropic energy is mainly limited by the field generated by the head. On the other hand, it is undesirable to increase the volume of the grains since this would increase the media noise.
Recently, a technique has been proposed in a Japanese Laid-Open Patent Application No.2001-56924 so as to avoid the problems of thermal instability. According to this proposed technique, the magnetic recording medium has two magnetic layers separated by a nonmagnetic layer made of Ru or the like, and the separated magnetic layers are antiferromagnetically coupled by an exchange coupling force so that the magnetic layers have antiparallel magnetizations.
FIG. 1 is a cross sectional view showing a magnetic recording medium employing this proposed technique. In FIG. 1, magnetic layers 11 and 13 are separated by an exchange coupling layer 12 which is made of Ru or the like, and magnetization directions of the magnetic layers 11 and 13 are antiparallel due to the exchange coupling force. By making the amounts of magnetization, that is, the saturation magnetization and thickness products, of the magnetic layers 11 and 13, unbalanced, a read head can detect a difference between the amounts of magnetization of the magnetic layers 13 and 11 as the recorded magnetization. Because the use of this difference between the amounts of magnetization in effect reduces the apparent thickness of the magnetic layer 13 which equivalently functions as the recording layer, it is possible to increase the linear recording density of the magnetic recording medium. In FIG. 1, a stacked structure 10 is made up of a substrate having an underlayer disposed thereon.
FIG. 1 also shows, conceptually, a magnetic particle 15. As shown in FIG. 1, the magnetizations above and below the exchange coupling layer 12 are antiferromagnetically coupled via the exchange coupling layer 12 and are antiparallel within one magnetic grain 15. As described above, when viewed from the read head, the amount of leakage magnetic field detected from the magnetic recording medium only amounts to that corresponding to an effective thickness te, due to the mutual cancellation effect between the magnetic layers 11 and 13. However, because the volume of the magnetic grain 15 is defined by V1, which corresponds to an actual thickness ta, it is possible to increase the apparent volume of the recording layer even though the magnetic recording medium in effect has a thin recording layer. For this reason, it is possible to realize a magnetic recording medium which is thermally stable.
By using the exchange coupling described above, it is possible to realize a thermally stable magnetic recording medium. Even though the conventional limit of the recording density was considered to be approximately 40 Gb/in2 due to reduced superparamagnetism, it has been confirmed that the recording density of the longitudinal magnetic recording can be increased to approximately 100 Gb/in2 using exchange coupling.
But in the case of the magnetic recording medium using exchange coupling, the present inventor has confirmed that the resolution may deteriorate and the nonlinear bit shift may increase from values which are predicted from the effective thickness te. A description will now be given of the problems generated in the magnetic recording medium using the exchange coupling, by referring to FIGS. 2, 3A, 3B, 4A and 4B.
FIG. 2 is a diagram showing a hysteresis loop for the magnetic recording medium shown in FIG. 1. In FIG. 2, the ordinate indicates the magnetization in arbitrary units, and the abscissa indicates the coercivity in arbitrary units. In addition, magnetization directions of the magnetic layers 11 and 13 are shown alongside the loop in FIG. 2. This loop is obtained using a measuring apparatus which requires several tens of minutes for the magnetic field to change, such as a vibrating sample type magnetometer. As may be seen from FIG. 2, the magnetization directions of the antiferromagnetically coupled magnetic layers 11 and 13 are antiparallel in a state ST1 or a state ST2, where these states ST1 and ST2 are residual magnetization states.
FIGS. 3A and 3B and FIGS. 4A and 4B are diagrams for generally explaining situations where bits are recorded on the magnetic recording medium shown in FIG. 1. In the case of the magnetic recording medium having the hysteresis loop shown in FIG. 2, the use of a material having magnetic anisotropy for the magnetic layer 11 increases the KuV/kT value which is a parameter of the thermal stability of the magnetic recording medium as a whole, and it is possible to obtain a high thermal stability, where Ku denotes a magnetic anisotropy constant, V denotes an average magnetic grain volume, k denotes a Boltzmann constant, and T denotes the temperature. The KuV/kT value is sometimes also referred to as the thermal stability factor.
Generally, it is known that a coercivity Hc of the magnetic recording medium is increased by a high-speed A.C. magnetic field. Such an increasing coercivity Hc is called a dynamic coercivity Hc′ or, simply dynamic Hc. The dynamic coercivity Hc′ determines an in-plane overwrite characteristic of the magnetic recording medium. It may easily be inferred that such a phenomenon also occurs physically in the magnetic layer 11.
FIGS. 3A and 3B and FIGS. 4A and 4B show the magnetization states which occur by the writing of a write head. FIG. 3A shows a case where the dynamic coercivity Hc′ of the magnetic layer 11 does not become large in a time region of a write process of the write head, where a magnetization inversion process α of the magnetic layer 11 is located in a negative magnetic field region. On the other hand, FIG. 4A shows a case where the dynamic coercivity Hc′ of the magnetic layer 11 becomes large in the time region of the write process of the write head. In other words, in the case shown in FIG. 4A, even when the hysteresis loop shown in FIG. 2 is obtained by changing the magnetic field by taking a sufficiently long time, the magnetization inversion process of the magnetic layer 11 may enter a positive magnetic field region as indicated by β if the dynamic coercivity Hc′ of the magnetic layer 11 increases in the time region of the write process of the write head. In FIGS. 3A and 4A, the ordinate indicates the magnetization in arbitrary units, and the abscissa indicates the magnetic field in arbitrary units.
FIG. 3B shows the formation of a magnetization transition region for the case shown in FIG. 3A, that is, for the case where the time dependency of the dynamic coercivity Hc′ of the magnetic layer 11 is not large and a magnetization inversion process L1 remains in the negative magnetic field region even for a high-speed change of the magnetic field. In FIG. 3B, it is assumed that a trailing edge 19 of the write head moves from the left to right. In the state shown in FIG. 3B, a region on the right side of the trailing edge 19 of the write head corresponds to a position below a write gap. When the magnetic field of the head is instantaneously inverted in the state shown in FIG. 3B, a transition region is written in the magnetic layer 13. In FIG. 3B, a dotted line conceptually shows the magnetic field from the head, and arrows indicate the direction of the magnetic field after the magnetic field of the head is inverted. In the magnetic layer 13, the region on the right side of the transition region corresponds to a magnetic field region B of the loop shown in FIG. 3A, and the recorded magnetization direction is to the right. On the other hand, in the magnetic layer 11, the magnetic field generated by the head in the region on the left side of the gap corresponds to a magnetic field region A of the loop shown in FIG. 3A, and the recorded magnetization direction of the magnetic layer 11 in the region on the right side of the gap is to the right as shown in FIG. 3B. Hence, as may be seen from FIG. 3B, the transition region is only formed in the magnetic layer 13 in the vicinity of the trailing edge 19 in the case of this magnetic recording medium.
On the other hand, when the time dependency of the dynamic coercivity Hc′ of the magnetic layer 11 is large, the hysteresis loop of the magnetic recording medium becomes as indicated by a solid line in FIG. 4A for the high-speed change of the magnetic field. FIG. 4B shows the formation of a magnetization transition region for such a magnetic recording medium. It is assumed for the sake of convenience that the write head used is the same as the write head used in FIG. 3B. In FIG. 4B, it is assumed that the trailing edge 19 of the write head moves from the left to right. In the state shown in FIG. 4B, a region on the right side of the trailing edge 19 of the write head corresponds to a position below the write gap. When the magnetic field of the head is instantaneously inverted in the state shown in FIG. 4B, a transition region {circle around (1)} is written in the magnetic layer 13. In FIG. 4B, a dotted line conceptually shows the magnetic field from the head, and arrows indicate the direction of the magnetic field after the magnetic field of the head is inverted. In the magnetic layer 13, the region on the right side of the transition region {circle around (1)} corresponds to a magnetic field region C of the loop shown in FIG. 4A, and the recorded magnetization direction is to the right. On the other hand, in the magnetic layer 11, the region on the left side of the transition region {circle around (1)} corresponds to the magnetic field range of the head from the region B to the region A, and the recorded magnetization direction of the magnetic layer 11 remains to the left. In the magnetic recording medium having the hysteresis loop shown in FIG. 4A, the dynamic coercivity Hc′ of the magnetic layer 11 is time dependent, and the dynamic coercivity Hc′ of the magnetic layer 11 increases in the high-speed write region of the magnetic storage apparatus. Thus, the magnetization inversion of the magnetic layer 11 that is observed when the vibrating sample type magnetometer or the like is used to make the measurement by taking a long time, changes from a position α′ to a position β during the high-speed write in FIG. 4A. For this reason, the magnetization of the magnetic layer 11 becomes complex compared to the magnetic recording medium having the hysteresis loop shown in FIG. 3A.
In other words, the magnetic field of the head indicated by the dotted line in FIG. 4B decreases towards a position on the left side of and further away from the trailing edge 19. When the magnetic field takes a value at the position indicated by β in FIG. 4A, a magnetization transition region {circle around (2)} is formed in the magnetic layer 11. As the position on the left side of the trailing edge 19 moves further away from the trailing edge 19, the magnetic field of the head decreases. In addition, since the region on the left side of the trailing edge 19 experiences a magnetic field in a direction opposite to that of the magnetic field indicated by the dotted line, a magnetization transition region {circle around (3)} having a stretch to a certain extent is formed by the inversion of the magnetic layer 11 indicated by a α″ in FIG. 4A.
In the magnetic layer 11, the magnetic field of the head acts in a region B shown in FIG. 4A, in a range from immediately below the magnetization transition region {circle around (1)} to the magnetization transition region {circle around (2)}. In addition, in the magnetic layer 11, the magnetic field of the head acts in a region A shown in FIG. 4A, in a range from the magnetization transition region {circle around (2)} to the magnetization transition region {circle around (3)}. The positional relationship of the magnetization transition regions {circle around (1)}, {circle around (2)} and {circle around (3)} varies depending on the magnetic characteristic of the magnetic recording medium, the data transfer rate of the magnetic storage apparatus such as the magnetic disk drive, and the magnetic field intensity generated by the head. But when the recording is carried out at approximately 40 Gb/in2, an interval between the magnetization transition regions {circle around (1)} and {circle around (2)} is approximately 20 nm which is narrow compared to a minimum bit interval of approximately 40 nm for the 40 Gb/in2 recording. On the other hand, an interval between the magnetization transition regions {circle around (1)} and {circle around (3)} is approximately 300 nm, and an interval between the magnetization transition regions {circle around (2)} and {circle around (3)} is considerably large compared to the minimum bit interval of 40 nm for the 40 Gb/in2 recording.
As is evident from the discussion above, no magnetization transition region is formed in the magnetic layer 11 in the vicinity of the trailing edge 19 as shown in FIG. 3B in the case of the magnetic recording medium having the hysteresis loop shown in FIG. 3A. But in the case of the magnetic recording medium having the hysteresis loop shown in FIG. 4A, the magnetization transition region is formed in the magnetic layer 11 in the vicinity of the trailing edge 19 as shown in FIG. 4B.
When recording the second bit on the magnetic recording medium, the write magnetic field of the head increases due to the magnetic field generated by the previously recorded first bit. As a result, a phenomenon occurs where the second bit is recorded at a position closer to the previously recorded first bit than an original position where the second bit should actually be recorded. This phenomenon is called non-linear transition shift (NLTS). The resolution deteriorates and the high-density recording capability deteriorates when the NLTS is large. Compared to the magnetic recording medium shown in FIGS. 3A and 3B, the magnetization transition region is additionally formed in the case of the magnetic recording medium shown in FIGS. 4A and 4B, thereby deteriorating the NLTS of the magnetic recording medium shown in FIGS. 4A and 4B. Because the magnetization transition region {circle around (3)} is far away from the magnetization transition region {circle around (2)}, the NLTS is virtually unaffected by the magnetization transition region {circle around (3)}.
The present inventor compared the magnetic recording medium of the type shown in FIG. 3A and the magnetic recording medium of the type shown in FIG. 4A, and confirmed that the resolution deteriorates by approximately 3% to 5% and the NLTS deteriorates by approximately 30% for the magnetic recording medium of the type shown in FIG. 4A. However, the present inventor also confirmed that, when a material having a large magnetic anisotropy is used for the magnetic layer 11 in the magnetic recording medium of the type shown in FIG. 4A, the thermal stability is better compared to that of the magnetic recording medium of the type shown in FIG. 3A.
Therefore, in the magnetic recording medium using the exchange coupling described above in conjunction with FIG. 1, there was a problem in that the thermal stability may improve depending on the behavior of the dynamic coercivity Hc′ of the magnetic layer 11, but the resolution and the NLTS may also deteriorate.